1. Field of the Invention
Embodiments of the present invention generally relate to satellite navigation receivers and more particularly to high sensitivity GPS receivers.
2. Description of the Related Art
Global positioning system (GPS) receivers typically use data from three or more orbiting satellites to determine navigational information such as position and velocity. There may be up to thirty functional satellites included in the GPS constellation, however, only a portion of those satellites may be visible to a particular GPS receiver at a given time. GPS satellites typically transmit information on two bands: the L1 band with a carrier frequency of approximately 1575.42 MHz and the L2 band with a carrier frequency of approximately 1227.40 MHz. Traditionally, only authorized users have been able to use data transmitted on the L2 band. In the future, civilian GPS signals may be transmitted on the L2 band and the L5 band (approximately 1176.45 MHz). However, low cost GPS receivers typically receive only on the one of these bands. The following descriptions use the L1 band to describe exemplary embodiments; however, other embodiments may be implemented using other GPS bands.
GPS satellites transmit data using a form of spread spectrum coding known as code division multiple access (CDMA). Each satellite is assigned a coarse acquisition (CA) code that resembles pseudo random noise and is unique to that satellite. Each satellite encodes data using the satellite's own CA code and transmits encoded data on the L1 carrier frequency. Thus, all satellites are simultaneously transmitting data on the shared carrier frequency. Each CA code consists of a sequence of 1023 “chips” where each chip is assigned a value of one or zero. The CA code is transmitted at a rate of 1.023 MHz; therefore, each chip period is approximately 0.977 us. Each satellite continually transmits a repeating pattern consisting of the satellite's own CA code. The GPS satellite may encode navigational or system data by inverting the transmitted CA code. CA code phase is the relationship of a CA code either to a reference clock or to other CA codes transmitted by other satellites. Although the CA code phase may be synchronized between satellites at the time of transmission, the CA codes may be received with differing delays at the GPS receiver due to different propagation times. Typically, a GPS receiver determines which CA codes are being received in order to determine which GPS satellites are in view.
There are many impediments to receiving signals from GPS satellites. GPS satellites orbit the earth with a period of approximately twelve hours. Thus, the signals from the satellites will always have some Doppler induced distortion. In addition, the L1 band is substantially a line-of-sight frequency band, i.e. the signals transmitted at those frequencies generally travel by line-of-sight, and may be easily blocked by buildings or terrain, and more prone to multipath distortion. Other interference sources that may impede signal reception are thermal noise, atmospheric propagation interference, and so forth. As is well-known, the GPS satellite signal is relatively weak compared to other radio frequency communication signals. Therefore, GPS satellite signal reception may be further hindered when the GPS receiver is inside of a building because the roof and walls of the structure may reduce the amount of satellite signal received.
There has always been a need for smaller and cheaper GPS receivers. As the size and cost of GPS receivers has decreased, manufacturers have integrated more GPS receivers into more products. For example, current GPS receivers are small enough to fit within cellular phones. However, still other product opportunities may present themselves as GPS receivers become even smaller. Furthermore, many consumer product opportunities are typically cost-sensitive. Therefore, relatively lower cost GPS receivers may be included in relatively more new consumer product designs.
As the foregoing illustrates, what is needed in the art is a high sensitivity GPS receiver with a relatively lower cost.